Performance charts and the POH

This element is about using the performance section of the POH/AFM — its charts, tables, and graphs — to produce defensible numbers for this flight. On the checkride the DPE hands you the book and watches you navigate to the right chart, interpolate the in-between values, and round and pad the result conservatively. The skill being tested is using the book, not reciting numbers from memory.

Book numbers are a floor, not a promise — each was measured under test conditions, in a new airplane, by a test pilot, on a dry, level, paved runway. Read the chart for the actual conditions, interpolate the in-between values, round every number to the conservative side, then add your own margin. Off the chart means no data — and no data is a no-go.

where the data lives

A standardized POH is organized into numbered sections. For performance work you live in a few of them:

• Section 5 — Performance — the charts and tables you compute from: takeoff distance, climb, cruise, and landing distance.
• Section 2 — Limitations — V-speeds, weight limits, and the CG envelope. These are the airplane's hard limits, covered in the limitations element (K4).
• Section 6 — Weight & Balance — empty weight, the equipment list, and a sample loading you work your numbers from.
• Supplements — at the back of the book, STC and optional-equipment data that overrides the base-book numbers for that installation. Check whether a supplement applies before you trust a baseline chart.

The book is not advisory. You may not operate the airplane outside the limitations in its approved AFM/POH §91.9, and before you fly you must determine runway lengths and compute takeoff and landing distance for the actual conditions §91.103(b). Those limitations trace back to certification flight testing under Part 23 — which is also why they can't be wished away.

the core performance charts

Most light-airplane POHs publish at least the four below; many publish more (crosswind component, stall speeds, range and endurance), so navigate the book rather than assume a fixed set. The common inputs are pressure altitude, temperature, weight, wind component, and runway surface — and pressure altitude and density altitude come from the density-altitude element (K2a); here they're simply chart inputs.

• Takeoff distance — ground roll, plus the total distance to clear a 50-ft obstacle.
• Climb — rate of climb, and the time, fuel, and distance to climb to a given altitude.
• Cruise — true airspeed and fuel burn at a chosen altitude and power setting.
• Landing distance — ground roll, plus the total over a 50-ft obstacle.

Read both takeoff figures: the ground roll (brake release to liftoff) and the total distance, which adds the air segment over a 50-ft obstacle. When there's a tree or fence off the end, compare the total against the runway available; on a long, clear runway, ground roll is what dominates.

interpolation discipline

Charts are published at round increments — pressure altitude every 1,000–2,000 ft, temperature every 10°C, weight in fixed steps. When your condition lands between two entries, interpolate linearly and show the work; don't eyeball it. If two inputs are both off-grid, interpolate one axis, then the other.

Say the table gives 1,000 ft of ground roll at PA 1,000 ft / 20°C and 1,200 ft at PA 2,000 ft / 30°C. For PA 1,500 ft / 25°C — halfway on both axes — the interpolated ground roll is halfway between, 1,100 ft.

Never run the line past the edge of the table. Extending a trend beyond the charted range isn't interpolation — it's guessing.

conservative-side rounding

Every reading is a small choice about which way to round, and the choice should always err toward less performance and more distance:

• Round inputs toward the worse case — pressure altitude and temperature up, weight up.
• Round distance outputs up — takeoff and landing distance to the next higher value.
• Round performance outputs down — rate of climb, cruise speed, and range to the next lower value.

The habit is simple: when in doubt, make the airplane look worse than the chart says, never better.

margin over book numbers

A book number is an honest read of an idealized airplane — factory-fresh, flown by a test pilot, on a perfect surface. Your trainer has hours on it and you are not a test pilot, so the book figure is a floor to build on, not a target to hit.

Add a personal margin on top of a conservative read. Common habits: add at least 50% to the computed takeoff or landing distance (multiply by 1.5), or require that your landing distance fit within roughly 60% of the runway available. These aren't regulations — they're personal minimums, and the DPE wants to hear that you apply one.

when the chart stops applying

Sometimes the right answer isn't a number — it's recognizing that you don't have one. A chart only describes the conditions it was built for:

• Off the charted range — past the highest pressure altitude, temperature, or weight on the table. The data ends there; don't extrapolate.
• Over gross weight — beyond max certificated weight, the chart and the certification behind it don't apply. There is no valid number to read; the fix is to offload weight.
• Contaminated or unpaved surface — the chart assumes dry, level pavement. Grass, snow, slush, standing water, and slope are unmodeled. Apply a published POH surface correction if one exists; if not, treat the distance as unknown and longer.
• Wind beyond what was tested — the demonstrated crosswind is the highest value shown during certification flight testing, not a Part 23 limitation. For chart purposes it marks the edge of tested data; past it, the airplane's behavior simply wasn't characterized.

In every case the discipline is the same: if your condition falls outside what the chart models, you don't have data — and no data is a no-go.

worked examples

Two reads, end to end. The chart values below are illustrative — always use the book for your airplane.

Scenario 1 — takeoff, interpolating both axes.

A Cessna 172 at 2,300 lb (max gross). Pressure altitude 5,000 ft, OAT 25°C, calm wind, dry level paved runway with 4,000 ft available. (Pressure altitude and density altitude come from the density-altitude element (K2a); here they're chart inputs.)

The takeoff chart brackets the conditions:

• PA 4,000 ft / 20°C → 1,600 ft over a 50-ft obstacle
• PA 4,000 ft / 30°C → 1,750 ft
• PA 6,000 ft / 20°C → 1,900 ft
• PA 6,000 ft / 30°C → 2,100 ft

Interpolate temperature (25°C is halfway) at each altitude: 4,000 ft → 1,675 ft; 6,000 ft → 2,000 ft. Then interpolate altitude (5,000 ft is halfway): (1,675 + 2,000) / 2 = 1,838 ft, round up to ~1,850 ft.

Add a 50% margin: 1,850 × 1.5 = 2,775 ft required. Against 4,000 ft available, it fits with room to spare. Go.

Scenario 2 — landing, off the chart's surface.

The same trainer, landing on a 2,500-ft runway of wet grass. The landing chart gives about 1,300 ft over a 50-ft obstacle for the weight and conditions — on dry, level pavement, which this runway is not.

Wet grass is unmodeled. If the POH published a grass or contaminated-surface correction you'd apply it, but this book publishes none — so the real landing distance is unknown and longer, not 1,300 ft.

With no valid number and only 2,500 ft available, the honest call is no-go — divert to a paved alternate. "Off the chart" isn't a math problem to force; it's the decision.